a) Field of the Invention
The present invention relates to a projector-type head lamp for motor vehicles, and more particularly to a projector-type head lamp which will not have an undesirable color fringe from chromatic aberration inavoidably taking place in this type of head lamp.
b) Description of the Prior Art
The well known projector-type head lamp for motor vehicles comprises a reflector with a spheroidal inner reflecting surface, a light source located at the first focus of the reflector, a shade having a cut-off edge in the vicinity of the second focus of the reflector, and a projection lens consisting of a single lens element, these being arranged in order. The illumination pattern at the shade is projected or focused on the roadway through the projection lens, but there develops at the light-dark border of the projected illumination pattern undesirable color fringing from chromatic aberration. To avoid such color fringing, some techniques or methods have been proposed to make less noticeable a color fringe by shifting or deflecting into the white zone the rays of light resulting in the color fringe from near the light-dark border. To this end, (1) U.S. Pat. No. 4,562,519 discloses a head lamp in which localized deflector elements are provided at the upper part and the lower part of the lens to produce a lateral dispersion and/or lowering of the light passing through these parts of the lens in order to reduce the effects of the chromatic aberration in the vicinity of the cut-off, and (2) U.S. Pat. No. 4,771,372 discloses a head lamp having means for at least reducing a color fringe otherwise present on the light-dark boundary of the light beam from chromatic aberration and including a correction element arrangeable in the path of the light beam and associated with the achromatic lens. In these conventional head lamps, the localized deflector elements or correction element are arranged integrally with or adjacent to the single lens element but the single lens element itself lacks the effects to deflect the rays of light forming the color fringe and reduce the color fringe. Hence, the localized deflector elements or correction element have to be manufactured with high precision, bearing in mind the relation with the achromatic lens. A further projector-type head lamp is known from the U.S. Pat. No. 4,100,594, which comprises a light source, a projection lens composed of first and second parts disposed on opposite sides of a plane, a single or a pair of masks disposed between the light source and the projection lens to define a cut-off in the projected beam. In this type of projector-type head lamp, color fringing is reduced by the use of a mask having separate front and rear edges or by splitting the projection lens into upper and lower halves, the arrangement in each case producing both a virtual and a real inverted mask images from the other half, thereby strongly colored rays are superimposed on the brightly illuminated area of the projected beam pattern where they have little effect on the overall color of the light while the least colored rays from the sharp cut-off edge and a region of reduced brightness adjacent to it. However, the mask forming such projector-type head lamp is formed having separate front and rear edges or the projection lens is formed as split in upper and lower halves, so that the optical system is complex and the mask and split projection lens have to be manufactured with high precision and have a correctly adjusted mutual relation between them.